How can advertisers learn about interest groups? about turtledove

[michaelkleber]

Yes, as long as the "URL information" is made aggregatable.

Certainly you should be able to use the aggregated reporting API to record your interest group and the domain of the publisher page, and then get a histogram of the top domains on which that interest group showed ads. But the counts in the histogram would include some noise, and insufficiently popular domains for the interest group wouldn't appear at all.

Instead of aggregating on the raw domain, you could process the URL — either in your bidding JS directly, or on your server as part of the contextual call that produces a contextual signal that becomes auction input. Then for each interest group you could get a histogram of the values you derived from the URL. That could be a coarser grouping than domain (for example, to cluster together a bunch of small sites with similar topics) or a finer grouping (for example, to break out pages on particular topics from a large heterogeneous site).